A weekly drought report shows that conditions have worsened in some key Midwest farming states where the corn harvest is more than halfway completed.

The U.S. Drought Monitor map released Thursday shows that 75 percent of Iowa, the nation's biggest corn producer, is enduring extreme or exceptional drought, which are the two worst classifications. That's up roughly 10 percentage points from the previous week.

The worse conditions won't make much difference to Iowa's already withered corn crop but it could affect crops such as winter wheat.

Nearly 98 percent of neighboring Nebraska still falls into the two most severe drought categories.

Overall drought conditions in the lower 48 states remained relatively unchanged.